Reference U.S. Pat. No. 3,769,497, by the same inventor, was found to suffer from certain deficiencies that prevented its use. The present invention overcomes these deficiencies in addition to introducing some new concepts. While the concepts and configurations discussed here may be used in other applications for liquid measurements, their application to urine measurement will be stressed here.
Urine production of patients is one of the vital signs customarily monitored in hospitals. Its value is not limited merely to indications of renal functions, but it is also quite useful to gain insight into liquid balance, perfusion, cardiac output, edema, etc. Clinically, it is important to know the exact volume of urine discharged and the continuum of urine production. The cessation of urine production may be a warning of loss of proper contact between the patient and the catheter, clogging with kidney stones or other obstructions, or kinking of the catheter, or the degradation or even failure of physiological functions.
Several ways now exist to achieve one or all of these monitoring functions. In all of these, as indeed with the subject invention, the patient is catheterized by the insertion of the first end of a flexible plastic tube in the urethra. The opposite end of the tube communicates with the measuring means. The measuring means most commonly used is a transparent plastic bag, where the meniscus within the bag is evaluated against graduations arranged along the bag to determine the amount of liquid collected. Reading of the bag requires frequent observation by medical personnel to assure a continuum of urine production. Also, it is difficult to determine the recent addition of urine to a partially filled bag. Typically, this might require the determination of small additions to large existing volumes. Also, these bags do not have alarm provisions. One cause for such an alarm would be the unexpected appearance of gross hematuria.
Another means for measuring such fluid is by weighing the collected discharge. This technique permits the use of alarms if the rate of discharge deviates beyond previously set limits. This approach suffers from the high cost of circuit components, which normally include a strain gage load cell and highly stable voltage supplies.
Although various electrical means of liquid level determination have been proposed, they have been rejected due to their failure to provide adequate patient safety. It is considered poor practice to electrocute a patient through the electrically conductive urine.